1. Field of the Invention
The present invention relates to an electrical angle-detecting apparatus for detecting the electrical angle of a synchronous motor and to a driving system of the synchronous motor using the electrical angle-detecting apparatus. The invention also pertains to a method of detecting the electrical angle of a synchronous motor.
2. Description of the Related Art
Systems and methods proposed for detecting the electrical angle of a synchronous motor include one for detecting rotational orientations of permanent magnets on a rotor with a hall effect element and one for applying a high-frequency voltage onto a winding to detect the position of a rotor (electrical angle) based on the waveform of the voltage in the winding (for example, JAPANESE PATENT PUBLICATION GAZETTE No. S58-37790).
The latter structure utilizes the fact that rotation of the rotor generates a reverse electromotive voltage in the synchronous motor having permanent magnets. The reverse electromotive voltage and thereby the orientation of the rotor are estimated based on the voltage applied onto the winding and the current actually flowing therethrough. In actual operation, a high-frequency detection voltage is laid upon a driving ac voltage applied onto the stator winding, and the electrical angle is detected by detecting the waveform of the voltage in the winding. The method of detecting the electrical angle in response to the variation in the inductance of the stator winding does not require any special sensor.
Motor-control systems using such technique do not need a special sensor for detecting the rotational orientation of the rotor and are thereby referred to as sensor-less control devices. Improved sensor-less control devices previously proposed have an internal arithmetic expression model for accurately detecting the electrical angle and correct the estimated position of the rotor based on the difference between the electric current calculated and the electric current observed (for example, `BRUSH-LESS DC MOTOR CONTROL SYSTEM WITH NO POSITIONING DETECTOR`, Masakane SHIKKO and Nobuyuki MATSUI, Material for 1990 Meeting on Semi-Conductor Power Conversion SEP-90-21).
In the conventional method of evaluating the electrical angle based on the reverse electromotive voltage, rotation of the rotor is essential for generating the reverse electromotive voltage, and the electrical angle can not be detected while the rotor stops rotation. A technique proposed for measuring the electrical angle under inactivating conditions of the rotor makes the electric current flow through a stator winding according to a predetermined pattern so as to rotate the rotor at random and generate a reverse electromotive voltage. In a system that a shaft of a motor is directly connected to an outside member, however, this technique may result in unexpected movements of the outside member. In an electric vehicle having a motor directly connected to wheels via gears, for example, the technique allows a slight driving force to be transmitted to the wheels. Such non-control state is extremely dangerous in the ever-changing conditions of driving and the road surface.
A variety of techniques proposed for the sensor-less control can not solve the problem arising under the inactivating conditions of the rotor. As long as that the rotor rotates at a predetermined or higher speed, the sensor-less control utilizing the reverse electromotive voltage is an excellent method of precisely detecting the electrical angle and thereby efficiently controlling the motor. A novel technique for estimating and detecting the electrical angle even while the rotor stops rotation or gradually increases the speed of rotation to a fixed level has thus been highly demanded.
There is a proposed method of determining the electrical angle of a synchronous motor with permanent magnets based on the inductance varied with the electrical angle of the synchronous motor (for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. H6-113585 and materials of the 64th and 74th Power Electronics Conferences). This method, however, determines the electrical angle only in the unit of 30 degrees or otherwise requires solution of complicated voltage equations for determination of the electrical angle at the precision of less than 30 degrees, thus not realizing the practical control.